The invention concerns a blind stitch sewing mechanism including a sewing head with a carrier, a needle shaft rotatably supported on the carrier and connected with a needle holder radially displaced from the needle shaft for holding a needle shaped arcuately about the axis of the needle shaft, a needle drive for reciprocating the needle back and forth, a loop catcher moveably supported on the carrier for receiving a thread loop formed upon the withdrawal of the needle through the work material, a loop catcher drive for movement of the loop catcher in timewise coordination with the movement of the needle, and a means for creating a relative movement between the sewing head and the work material.
A blind stitch sewing machine of the above named kind is for example known from DE-PS 1106153. In the solution there described the work material, with the help of a material bender arranged below the work material support surface, is locally pressed upwardly with each stitch so that a small section of the work material comes into the curved movement path of the needle. The movement path of the needle is perpendicular to the transport direction of the work material. The loop catcher must execute a relatively complex movement in order to so hold the loop stemming from the previous stitch that the needle in the immediately following stitch extends through the loop. A further disadvantage is that this blind stitch machine is only useable for relatively thin work material which is insertable between the needle and the material bender. It is therefore a prerequisite that the work material is accessible at its underside in order to provide the construction components making up the material bender.
There are workpieces which, because of their measurement and shape, can not be inserted into a blind stitch sewing machine of the previously mentioned kind. In order to be able to create sewn seams at desired spots in three dimensionally shaped workpieces independently of their size, a sewing machine is already known from DE 19751001 in which an up and down moving straight needle and an up and down moving straight line catcher in a common movement plane perpendicular to the sewing direction are inclined by a sharp angle to one another so that their movement paths intersect one another below the workpiece. The catcher grabs the loop formed upon the withdrawing of the needle below the workpiece and pulls it upwardly to the upper side of the workpiece where it positions the loop in the movement direction of the sewing machine, that is in the sewing direction. During the next oscillation the newly formed loop is pulled through this earlier loop, and so forth. The sewing head can be arranged on a robot by means of which it is guided over the work material surface. The sewing machine requires no construction components arranged below the workpiece and it is therefore useable independently of the size and shape of the workpiece. The support surface for the workpiece must of all things have a gap in the area of the seam to be formed so that the needle and catcher can penetrate through the workpiece and extend out of the underside of the workpiece.
In the manufacture of fiber reinforced composite materials fiber mats have to be sewn together before the fiber mats lying in forms are cast with plastic. In this case it is desirable that the mats not be moved again after the sewing, that is that they for example be sewn in the form. This is not possible with the sewing apparatus according to DE-PS 110653 or DE 19751011.
The invention has as its object the provision of a blind stitch sewing apparatus of the previously mentioned kind by way of which a sewn seam can be formed in workpieces of any chosen size, without the seam forming work tools having to escape from the opposite side of the workpiece or without the opposite side of the workpiece having to be accessible.
The above object is solved by the invention in that the thread catcher is supported for a back and forth movement perpendicular to the movement path of the needle and in that the means for creating the relative movement between the sewing head and a work material is so formed that the formed sewn seam runs at least nearly in the direction of the movement path of the needle.
In the inventive solution the circular arc shaped needle dips into the work material which can for example have a thickness from 10 to 15 mm. The needlepoint then again protrudes from the work material at the insertion side of the work material without the needle touching the side of the workpiece opposite to the insertion side. The work material therefore remains lying on the under support and is not lifted or deflected by a material bender or by a part moving out of its underside. The loop catcher executing a simple pivotal movement holds the loop during the reverse movement of the needle and carries it along for the distance of the stitch length, after the needle has left the work material and the sewing head moves relative to the work material. The loop catcher holds the loop until the needle point during the immediately following stitch passes through the loop.
Since the needle and loop catcher each execute only a pivotal movement about a rigidly positioned axis the construction of the inventive sewing apparatus is extraordinarily simple. Since in the inventive blind stitching sewing apparatus the side of the workpiece opposite the stitch insertion side remains undisturbed the support for the workpiece may be of any chosen shape. It is especially possible to lay mats of fiber material to be sewed into a casting form prior to the sewing, which casting form after the completion of the sewing process can then be filled with cast plastic material. In this way the sewn fiber material needed for the making of the composite material need not be moved after the sewing operation.
Preferably a work material holddown is arranged on the carrier, which holddown has a slot corresponding to the movement path of the needle, to avoid the work material being taken along with the needle during the withdrawal of the needle from the work material. To be able to regulate the penetration depth of the needle the work holddown is advantageously arranged on the carrier in such a way as to be adjustable in height.
Thanks to the simple movement executed by the loop catcher, the loop catcher can be formed as a simple thorn fastened to a pivotal lever, which lever in turn is pivotally supported on the carrier and is connected with the catcher drive.
According to a first embodiment the needle drive has a drive motor, reversible as to its direction of rotation, the output shaft of which needle drive is coupled with the needle shaft through a drive mechanism. One such reversible drive motor is for example a step motor. The drive mechanism can include a toothed belt which meshes with a toothed pulley on the output shaft of the drive motor and with a toothed pulley located on the needle shaft. This kind of drive permits a variable construction of the sewing head, since the toothed belt requires only a small amount of space and, for example, a very lean carrier can be made, at the free end of which the needle and the loop catcher are arranged, in order to be able to sew even in narrow deeply lying hollow spaces.
In another embodiment of the invention the needle drive has a continuously rotating motor which drives a toothed segment pivotally supported on the carrier through an eccentric or crank drive, which toothed segment meshes with a gear arranged on the needle shaft. The toothed segment is reciprocated back and forth by the eccentric drive and in keeping with this likewise moves the needle shaft and the needle back and forth.
The catcher drive is advantageously controlled a cam disk or plate, for example located on the eccentric shaft. This offers the possibility of shaping the movement of the loop catcher in any desired way within a stitch cycle, while the needle movement in generally so occurs that the needle requires the same amount of time for each forward and reverse movement of the needle in the work material. However, the loop catcher can also be directly driven by means of a reversible drive motor.
The advantages of the inventive sewing apparatus especially come to light if the sewing head is arranged on a robot which moves the sewing head over very large workpiece surfaces in accordance with a pre-given program. Such a robot has to be built relatively massively in order to be able to precisely move the sewing head. Since it is difficult at high sewing speeds to stop the robot and start it again with each stitch, and the sewing head may not be moved relative to the workpiece at least so long as the needle is stuck entirely in the workpiece, it is advantageous to move the robot continuously and to support the carrier in a frame of the sewing head so as to be translatorially adjustable, so that it can be moved back and forth parallel to the relative movement of the sewing head and the work material. Similarly to the known needle transport of a sewing machine in which the needle is moved along with the workpiece for so long as it inserted into the workpiece, in this case the carrier remains unmovable relative to the workpiece for so long as the needle resides in the workpiece while the frame of the robot is continuously moved forwardly in the sewing direction. Because of the fact that the movement path of the sewing head is parallel to the movement path of the needle the carrier can also be moved earlier relative to the workpiece after the needle point during the withdrawal of the needle from the workpiece has passed beyond the deepest point of the needle path inside of the work material. This holds true at least if the workpiece itself is sufficiently flexible.
The thread loop formed at the needlepoint upon withdrawal of the needle is relatively small. To assure that the loop catcher reliably attaches to the loop it must be guaranteed that the needle independently of the forces exerted by the workpiece onto the needle constantly takes on the same position relative to the loop catcher. For this reason it is proposed according with the invention that above the workpiece holddown a guide is arranged for an extent of the needle in the neighborhood of the needle point, which guide after the insertion of the needle into the workpiece secures the later escaping forward needle area against a lateral deflection. Preferably this guide is formed as a circular groove in a roll, so that the needle is secured against lateral deflection by the walls of the groove.
Further problems can arise with very rigid or hard workpieces. If the needle point encounters such a hard or tough material there exist a danger that the needle will be heavily bent, by the high resistance at the point, between the point and the end of the needle opposite to the point, that is will be deflated outwardly from its circular arc shaped curvature. This stress increases the danger of a needle breakage. For this reason it is proposed according to the invention that the arcuately curved needle has associated with it a needle support which supports the needle at its convex outer side along at least a portion of the needle movement. Thereby an outward bending of the needle and associated material fatigue is avoided. The needle support can be formed in a simple way on a roll which is rotatably supported for movement about an axis parallel to the curvature axis of the needle and which has a circumferential groove for receiving the needle, so that the needle is guided nearly friction free on the roll and at the same time is supported by the roll.
To be able one hand to support a middle area between the needle point and the end held by the needle holder, and on the other hand to not hinder the pivotal movement of the needle, it is advantageous if the roll is adjustable between a supporting position in which it lies against the needle and an alternative position at which it has a spacing from the needle and does not hinder the pivotal movement of the needle holder. The adjustment of the roll can take place in a simple way in that it is arranged on a pivotal lever coupled with the sewing machine drive so that it is pivotal in timewise coordination with the needle movement between a first position corresponding to the supporting position of the roll and a position corresponding to the alternate position of the roll.
Whenever a sewn seam section is finished and the sewing head is to be moved relative to the work material for the beginning of a new seam, the thread or threads have to be cut. For this different thread cutters are known. The arrangement of a thread cutter near the needle movement path is however associated with great difficulties because of space problems. According to the invention this problem is solved in an especially space saving way in that the work material holddown has a foot plate for engaging the work material in which foot plate a slot for the needle is formed so that at least one of the slot longitudinal edges is formed as a first cutting edge, in that a counter knife is arranged parallel to and above the foot plate with a second cutting edge, and in that the foot plate and the counter knife are movable relative to one another parallel to the upper surface of the foot plate. This allows the thread to be cut very close to the seam end. Preferably the foot plate and the counter knife are pivotal relative to one another about an axis perpendicular to the upper surface of the foot plate. If the cutting edge of the counter knife is positioned close to the one longitudinal edge of the slot for the passage of the needle and the oppositely lying edge of the slot forms the other cutting edge a very small pivotal movement is sufficient to bring the cutting edges along their entire length into engagement with one another since the slot is relatively small. In the case of this embodiment the counter knife or the foot plate is fastened in a tube while the other part (foot plate, counter knife) is fastened to a rod extending through the tube, which rod is coupled with a pivotal drive.